The performance of scientific instruments, such as mass spectrometers, which operate under vacuum conditions with the ions of interest produced externally at atmospheric pressure are profoundly affected by the efficiency of ion transfer between the atmosphere and vacuum regions of the instrument. As transfer efficiency increases, loss of ions produced from the sample of interest is reduced, and the number of informative ions that enter the instrument is increased. This can result in increased speed of analysis, resolution, and sensitivity of the instrument.
Among the most rudimentary atmosphere-vacuum interfaces is a small orifice in the first vacuum chamber evacuated by a roughing pump to pressures of about 1-10 Ton. The pumping speed of typical roughing pumps is usually a few liters/s, which places a limit on the diameter of the orifice of typically less than 0.5 mm. Ion beams created this way are usually poorly collimated, so that the beam diameter quickly increases downstream of the orifice. To avoid destroying the ion beam and incurring ion losses, a skimmer electrode is typically positioned 4-7 mm downstream of the orifice to provide a means for ion passage further into the next higher vacuum stage of the instrument, as described, for example, in a publication by Fenn, “Mass spectrometric implications of high-pressure ion sources,” Int. J. Mass Spectrom. 2000, 200: 459-478.
The first atmosphere-vacuum interfaces for coupling electrospray ionization (ESI) sources to mass spectrometers were designed on this principle, and some mass spectrometer manufacturers still use this design with little or no modifications. One disadvantage of this rudimentary interface is the absence of an efficient means to supply heat to the small charged droplets produced by ESI and the associated heavily solvated ions after they have entrained in the supersonic jet formed by gas expansion into the vacuum.
The effects of adiabatic expansion cooling can be counteracted to some extent by creating a declustering potential between the orifice and the skimmer. However, the amplitude of the declustering voltage cannot be very large because it will induce dissociation of the already desolvated ions. Other modifications to this rudimentary interface previously proposed to improve the ion desolvation process include introducing a counter flow of heated gas (sometimes referred to as a heated gas curtain), heating the entire interface, and installing a heated laminar flow chamber (particle discriminator interface, PDI) in front of the orifice. However, these modifications are expensive, and/or frequently of very limited efficiency, often requiring precise controls for optimization of temperature and gas flows for the particular analyte and solvent system. Such controls are needed to insure complete desolvation and to prevent a decrease in sensitivity from ions being swept away at gas flow rates that are too high.
One efficient solution to improving the ion desolvation process without the need for precise gas flow control is described in co-owned U.S. Pat. No. 4,977,320 to Chowdury, et al., (hereinafter, “Chowdury”), entitled “Electrospray Ionization Mass Spectrometer with New Features,” which issued on Dec. 11, 1990. In the method disclosed by Chowdury, solvated ions formed by an electrospray ionization of an analyte solution at atmospheric pressure were introduced into a first vacuum chamber of a mass spectrometer through a metal capillary heated to, for example, about 85° C. The capillary in Chowdury is about 0.5 mm in diameter and of 203 mm in length, and projects into the first vacuum chamber 21 of the mass spectrometer. Chowdury further discloses that heating of the capillary tube causes evaporation of the droplets and desolvation of the resulting molecular ions of interest for analysis. Such ion interfaces containing a heated metal capillary or an array of heated capillaries instead of a simple orifice have since became widely adopted by mass spectrometry manufacturers and researchers, especially when high flow-rate ESI ion sources are coupled to mass spectrometers.
With the advent of nano-flow ESI ion sources, or low flow-rate electrospray ionization sources, the sensitivity of mass spectrometers coupled to on-line chromatography has dramatically increased (see, e.g., U.S. Pat. No. 5,788,166 to Valaskovic, et al., entitled “Electrospray ionization source and method of using the same,” issued Aug. 4, 1998). Nano-flow ESI emitters can potentially provide better conditions for sample ionization and, ultimately, higher ionization efficiency than the standard electrospray sources based on the heated metal capillary as described in Chowdury. However, little optimization has been made to ion interfaces that operate with nano-flow ESI sources to increase the efficiency of ion transfer between the atmosphere and the vacuum interface of a mass spectrometer.
Accordingly, there is still a need for a method and apparatus for improving the transfer of ions from atmosphere into a vacuum region of a mass spectrometer, particularly for mass spectrometers for coupling nano-flow ESI ion sources thereto.